Thomas K. Meehan

THE COSMIC/FORMOSAT-3 MISSION : Early Results

The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean. On 15 April 2006, the joint Taiwan-U.S. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit. After launch the satellites were gradually deployed to their final orbits at 800 km, a process that took about 17 months. During the early weeks of the deployment, the satellites were spaced closely, offering a unique opportunity to verify the high precision of RO measurements. As of September 2007, COSMIC is providing about 2000 RO soundings per day to support the research and operational communities. COSMIC RO dat...

GPS Sounding of the Atmosphere from Low Earth Orbit: Preliminary Results

Abstract This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earth-orbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dual-frequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides informat...

Initial Results of Radio Occultation Observations of Earth's Atmosphere Using the Global Positioning System

Recent radio occultation measurements using Global Positioning System satellite transmitters and an orbiting receiver have provided a globally distributed set of high-resolution atmospheric profiles, suggesting that the technique may make a significant contribution to global change and weather prediction programs. Biases in occultation temperatures relative to radiosonde and model data are about 1 kelvin or less in the tropics and are generally less than 0.5 kelvin at higher latitudes. Data quality is sufficient to quantify significant model errors in remote regions. Temperature profiles also reveal either an equatorial Rossby-gravity or an inertio-gravity wave. Such waves provide a fundamental source of momentum for the stratospheric circulation.

System and method for measuring ocean surface currents at locations remote from land masses using synthetic aperture radar

This is a system for measuring ocean surface currents from an airborne platform. A radar system having two spaced antennas wherein one antenna is driven and return signals from the ocean surface are detected by both antennas is employed to get raw ocean current data which is saved for later processing. There are a pair of GPS systems including a first antenna carried by the platform at a first location and a second antenna carried by the platform at a second location displaced from the first antenna for determining the position of the antennas from signals from orbiting GPS navigational satellites. This data is also saved for later processing. The saved data is subsequently processed by a ground-based computer system to determine the position, orientation, and velocity of the platform as well as to derive measurements of currents on the ocean surface.

Initial Results of GPS-LEO Occultation Measurements of Earth’s Atmosphere Obtained with the GPS-MET Experiment

The radio occultation technique, which has been repeatedly proven for planetary atmospheres, was first utilized to observe Earth’s atmosphere by the GPS-MET experiment (launched in April 1995), in which a high performance GPS receiver was placed into a low-Earth orbit. During certain phases of the mission, more than 100 occultations per day are acquired. A subset of this occultation data is analyzed and temperature in the neutral atmosphere and electron profiles in the ionosphere are obtained. Comparing about 100 GPS-MET retrievals to accurate meteorological analyses obtained from the European Center for Medium-range Weather Forecasting at heights between 5–30 km, temperature differences display biases of less than 0.5K and standard deviations of 1–2K in the northern hemisphere, where the model is expected to be most accurate. Furthermore, electron density profiles obtained for different geodetic locations and times show the main features that are expected in the ionosphere.

Direct Signal Enhanced Semicodeless Processing of GNSS Surface-Reflected Signals

This paper presents an aircraft demonstration of direct-signal enhanced semicodeless processing of global navigation satellite systems (GNSS) signals reflected from the Earths surface. Comparisons are made between this new method and an interferometric approach to GNSS reflectometry. Results show that this technique produces waveforms with greater signal-to-noise compared with the interferometric approach for all GNSS signals currently in use or planned for the near future. Alternatively, the semicodeless technique can have similar performance with smaller antennas for lower hardware costs. The semicodeless approach also has the advantage that different signals along with their different surface spatial resolutions are processed separately, each signals coherent integration time can be optimized, and ground/aircraft experiments and tests are free of spurious signals. The signal processing demands of the semicodeless approach are shown to be proportional to the number of signal components processed when integrated with a GNSS precise orbit determination (POD) receiver.

Operational aspects of CASA UNO '88-the first large scale international GPS geodetic network

For three weeks, from January 18 to February 5, 1988, scientists and engineers from 13 countries and 30 international agencies and institutions cooperated in the most extensive GPS (Global Positioning System) field campaign, and the largest geodynamics experiment, in the world to date. This collaborative experiment concentrated GPS receivers in Central and South America. the predicted rates of motions are on the order of 5-10 cm/yr. Global coverage of GPS observations spanned 220 degrees of longitude and 125 degrees of latitude using a total of 43 GPS receivers. The experiment was the first civilian effort at implementing an extended international GPS satellite tracking network. Covariance analyses incorporating the extended tracking network predicted significant improvement in precise orbit determination, allowing accurate long-baseline geodesy in the science areas. >

Precise orbit determination for COSMIC satellites using GPS data from two on-board antennas

Each of the six COSMIC satellites carries one GPS receiver with two antennas for orbit determination (OD) and another two antennas for radio occultation. In this paper we describe the features of the GPS data associated with COSMIC satellite attitude. Method and results of the precise orbit determination for the COSMIC satellites using the tracking data from both OD GPS antennas are presented. Special issues in satellite orbit determination with multiple GPS antennas are discussed. Orbit precision is evaluated and the effects of multipath errors are investigated.

Backpropagation Processing of GPS Radio Occultation Data

We provide an assessment of the backpropagation (BP) method for processing GPS radio occultations using simulations as well as recent data from CHAMP and SAC-C. It is found that BP gives improved retrievals over the standard Doppler technique, even when multipath ambiguities are not completely removed. In addition, by being an amplitude-weighted algorithm, BP is robust in the presence of receiver errors that arise when signals with low SNR are tracked.

Use of the L2C signal for inversions of GPS radio occultation data in the neutral atmosphere

Results from processing FORMOSAT-3/COSMIC radio occultations (RO) with the new GPS L2C signal acquired both in phase locked loop (PLL) and open loop (OL) modes are presented. Analysis of L2P, L2C, and L1CA signals acquired in PLL mode shows that in the presence of strong ionospheric scintillation not only L2P tracking, but also L1CA tracking often fails, while L2C tracking is most stable. The use of L2C improves current RO processing in the neutral atmosphere mainly by increasing the number of processed occultations (due to significant reduction in the number of L2 tracking failures) and marginally by a reduction in noise in statistics. The latter is due to the combination of reduced L2C noise (compared to L2P) and increased L1CA noise in those occultations where L2P would have failed. This result suggests application of OL tracking for L1CA and L2C signals throughout an entire occultation to optimally acquire RO data. Two methods of concurrent processing of L1CA and L2C RO signals are considered. Based on testing of individual occultations, these methods allow: (1) reduction in uncertainty of bending angles retrieved by wave optics in the lower troposphere and (2) reduction in small-scale residual errors of the ionospheric correction in the stratosphere.